In solid-state imaging devices used for CCDs, CMOS image sensors, and other image sensors, so-called vertical transistors have been practically used as switching elements for reading of a signal charge from a photodiode (PD).
The vertical transistors each include an electrode (a gate electrode) embedded in a depth (thickness) direction in a semiconductor substrate made of silicon (Si) or any other semiconductor. Using the vertical transistor makes it possible to read out a signal charge from a photodiode that is disposed in a deep region in the semiconductor substrate. Moreover, using the vertical transistor also makes it possible to relax a surface electric field of the photodiode. Further, it is possible to stack a plurality of photodiodes, thereby increasing saturation capacity of a pixel.
For example, Patent Literature 1 discloses a configuration in which two photodiodes are stacked in a semiconductor substrate and vertical transistors as mentioned above are provided for the respective photodiodes. In this configuration, signal charges of the photodiodes are read out through corresponding vertical transistors.